In the furniture industry, and related industries, a wide range of substrates must be adhered, including wood; metal, such as cold-rolled steel and aluminum; fabric; paper; leather; foam; plastic, such as polyvinylchloride, polystyrene, polyethylene, polypropylene, and acrylonitrile-butadiene-styrene (ABS), fiberglass, and materials used to construct high pressure laminates, for example, for counter tops.
As used herein a contact adhesive refers to an adhesive which must be applied to both substrates to be joined and allowed some time to dry, typically up to 24 hours before the two substrates are pushed together. Once the substrates are pushed together, the bond forms very quickly and it is usually not necessary to apply pressure for a long time. Some contact adhesives are able to provide bonded substrates that exhibit handling strengths within 20 seconds to 5 minutes after application to the substrates. That is, a bond of sufficient strength is formed to hold the substrates together and resist subsequent forces on the fresh bond-line which might cause failure. Adhesives that provide the desired handling strength soon after application and bonding are typically referred to as “fast-set” or “fast-setting” adhesives. To determine whether an adhesive is a fast-set adhesive, a pinch bond test may be utilized.
Pinch bonds, or knife-edge bonds, are used to evaluate if an adhesive formulation exhibits the desired handling strength characteristics that in turn enable immediate handling and processing during manufacture of foam rubber cushions used in furniture. Such bonds require more strength build-up than needed to form a typical bond since the former results in a bond-line under stress once the bond is made whereas the latter does not.
In the past, foam and furniture manufacturing has been dominated by one part contact adhesives dissolved or dispersed in organic solvents, such as chlorinated solvents and low flash point organic solvents. Such one part contact adhesives are conveniently able to be applied using a single source (i.e., container). However, there has been a desire to shift from organic, solvent-based adhesive compositions to aqueous-based or aqueous-dispersed adhesive compositions for environmental reasons.
It is desirable to find an aqueous-based adhesive that can be used as a fast-set adhesive. Attempts to provide them in one part form, however, have experienced only gradual industry acceptance since they have longer dry times than conventional organic, solvent-based adhesives, as well as a relatively slow rate of strength build. To overcome such limitations, two part (i.e., co-sprayed from two separate containers) aqueous-dispersed adhesive systems have been developed that demonstrate high adhesive strength within seconds of spraying. The adhesive composition is one part of the two-part system. An external coagulant, such as citric acid, lactic acid, acetic acid, or zinc sulfate, is typically used as the second part in a predetermined ratio with respect to the first part. Such two part adhesive systems, however, are not entirely satisfactory. The co-spraying equipment is expensive, the equipment requires maintenance, and the ratio of the two parts (the coagulant and the adhesive composition) must be monitored during application.
U.S. Pat. No. 6,086,997 to Patel et al. describe a storage stable, fast-setting, one part, aqueous contact adhesive composition comprising an adhesive component and boric acid. The boric acid is utilized as an internal coagulant and can be added in the form of a solution, or it can be generated in-situ. The adhesive component comprises at least one polychloroprene. The adhesive component may optionally comprise a mixture of polychloroprene and natural rubber, synthetic rubber, or combinations thereof. The adhesive component is substantially free (i.e., contains 5% by weight or less) of acrylates and can also contain an amino acid as an internal coagulant, such as glycine. It is disclosed that the addition of boric acid enables the pH of the polychloroprene adhesive composition to be lowered while still maintaining good shelf life. “Shelf life” is defined as the time period after which the aqueous composition has substantially coagulated, congealed, curdled, separated, settled, or formed non-easily mixable or non-easily dispersible layers so that it may not be usefully or readily applied as a homogeneous, uniform liquid blend by spray-coating methods. That is, compositions that have good shelf life are storage stable. “Storage stable,” is defined as meaning the aqueous compositions have a shelf life of greater than about four months when stored at room temperature (25° C. S.T.P.). “Fast-setting,” is described as referring to an adhesive composition that develops strength sufficient to form a bond when finger pressure is applied (i.e., a finger bond) in less than about 10 minutes after application of the adhesive to the substrate to be bonded. For some applications, e.g., bonding foam in the furniture industry, fast-setting adhesives desirably develop a finger bond within 90 seconds or less after application. The compositions are disclosed to be fast-setting due to the low pH of the compositions, which is preferably in the range of about 7 to about 9.5. The one part, fast-setting, storage stable, aqueous adhesive composition may be applied by spraying from one container.
U.S. Pat. No. 5,543,455 to Shah describes a waterborne adhesive comprising an aqueous emulsion of A) between about 50 and about 80 weight percent solids (based on A plus B) of an emulsified acrylic polymer, said acrylic polymer having an acid number of between about 5 and about 50, and an N-methylol acrylamide content of between about 0.5 and about 5 wt % based on monomer solids; B) between about 20 and about 50 weight percent solids (based on A plus B) of a latex of an elastomer, eg., rubber latexes such as natural rubber, neoprene, etc., and C) anionic surfactant in amount sufficient to stabilize the aqueous dispersion of A) and B). It is disclosed that N-methylol acrylamide is an essential monomer. The elastomer latex B) is said to be necessary for good bonding to be achieved as the acrylic resin alone does not give good bonding as a latex adhesive. It is also taught that contact adhesives based primarily on neoprene do not give good high-temperature adhesion and, accordingly, the acrylic polymer A) is used at a level of between about 50 and about 80 wt % and the elastomer latex B) is used at between about 20 and about 50 wt % (based on A plus B). The final pH of the waterborne composition is disclosed to be between about 7 and about 11.
EP 2246403 A1 to Motzet et al. describe floor or contact adhesives which are provided as a water-borne formulation comprising: a cross-linkable binder resin having a glass transition temperature (Tg) of less than +10° C.; optionally, a crosslinking agent; and, optionally a tackifier and/or a plasticiser; wherein the volatile organic compound (VOC) level of said formulation is less than 0.5% by weight. It is stated the cross-linkable binder resin should preferably have a glass transition temperature (Tg) in the range from +10° C. to −90° C. and furthermore should preferably comprise a carbonyl functional (meth)acrylate or vinyl copolymer prepared from a monomer mixture comprising alkyl (meth)acrylate ester monomers and/or vinyl monomers, and diacetonacrylamide (DAAM) monomers. Acrylic-based, carbonyl-functional binder emulsions having pH values of 6.9 and greater are exemplified. These were used to prepare adhesive compositions having pH values of 7.2 or greater.
A one-part laminating adhesive composition based on an acrylic polymer prepared by a “core-shell” emulsion polymerization process is disclosed in U.S. Pat. No. 4,948,822 to Iovine and Walker. The adhesive composition is based on an acrylic copolymer comprising a core and a shell in which a latently reactive, functional comonomer (for example, glycidyl methacrylate) is placed in the core and a second functional comonomer reactive with the first comonomer is placed in the shell. Monomers useful for preparing the acrylic polymer employed in the adhesives described therein are selected to provide a core-shell polymer wherein both the core and shell polymer will have a Tg of about −10 to −35° C. The weight ratio of core to shell monomers employed in preparing the polymer is stated to be in the range from about 2:1 to 5:1 (67-83 wt %). Typically, the adhesive is coated on a film and allowed to dry at room temperature (or dried at moderate heat). The adhesive coated film is then laminated to a desired substrate, for example, a corona treated polyethylene or polypropylene film or other lamina by passing through a “hot nip” roller. The resultant laminate is stated to form an immediate bond which gains strength on room temperature standing resulting from the polymeric hardening or curing. It is understood that the heat from the “nip” step employed in the laminating process is sufficient to permit the functional comonomers to react with one another and thereby to initiate hardening, ionic bonding or crosslinking of the polymer. The core-shell technology is used to effectively separate the reactive functional comonomers until such time as reactivity is desired.
The prior art does not provide an aqueous-based, one part adhesive that is free of various additives such as coagulants and storage stabilizers, free, or at least substantially free (e.g., less than 30 wt %, or less than 15 wt %, or even less than 5 wt %), of natural rubbers and halogenated polymers such as polychloroprene, that is fast-setting at room temperature, that can be applied to either one or both substrates to be bonded, and that provides bonded substrates that initially can be repositioned.